If you want it posted as your nostr identity, it's non-trivial because the service will have to have your nostr private key.

Ideally, I think you'd run such a service yourself that checks your twitter/insta for new posts and posts to nostr.

Next best thing is maybe a browser extension.

Third best thing is a website you visit that connects to twitter/instagram/nostr, you post there, and it posts to all of them.